Ozone, which causes photochemical smog, is produced by a photochemical reaction of HC and NOx contained in exhaust gases from automobiles and factories. Therefore, reducing the amount of emissions of HC and NOx from automobiles is an efficient way to suppress the production of ozone and the occurrence of photochemical smog. Also, purifying ozone in the atmospheric air directly can be one of the ways to prevent the occurrence of photochemical smog. By purifying ozone as a product as well as reducing the amount of emissions of HC and NOx as reactants, the occurrence of photochemical smog can be prevented more effectively. In this respect, an automobile including an air-purifying device for a vehicle capable of directly purifying ozone in the atmospheric air has been put into practical use in some places including California in the United States of America. This air-purifying device for a vehicle, particularly, is called a DOR (Direct Ozone Reduction) system.
Previously-used air-purifying device for a vehicle (a DOR system), in which National Publication of International Patent Application No. 2003-515442 represents, is the one that uses metal oxide such as manganese dioxide as a catalyst. By coating a catalyst made of metal oxide on a radiator into which air is delivered during travel of a vehicle, ozone in the atmospheric air is degraded and purified by the catalyst.
It has been known that not only metal oxide catalyst such as manganese dioxide but also activated carbon have a function for purifying ozone. By activated carbon, ozone is converted into carbon dioxide by reaction with activated carbon itself. Since this reaction occurs at ambient temperature, it can be said that activated carbon has an advantage in a purification condition over the metal oxide catalyst which purifies ozone at a higher temperature than the ambient temperature. Moreover, activated carbon has countless fine pores and its surface area per unit volume is quite large. Therefore, it has many chances to contact with ozone in the atmospheric air and thus has high ozone purification performance per unit volume. In addition, the fine pores of the activated carbon have an effect which lowers activation energy for the conversion of ozone to radical oxygen by electron donation from carbon (a capillary condensation effect). With the capillary condensation effect, the degradation of ozone in the fine pores of the activated carbon is accelerated.
However, at present, none of those air-purifying devices for a vehicle with an ozone purifier including the activated carbon have been put into practical use. For there becomes a problem where the activated carbon is used as an ozone purifier, its ozone purifying function is easily deteriorated. The reason for this is particulate matter (hereinafter called “PM” or “PMs”) contained in the atmospheric air. Invading PM into the fine pores of the activated carbon results in a clogging, which leads to a significant decrease in substantive surface area of the activated carbon, that is, surface area capable of contacting with ozone. PMs include the ones which are emitted from vehicles such as diesel vehicles and the ones from soil. The clogging of the activated carbon is caused by the former with small particle size. Under an environment with automobiles, naturally, the former level increases. Therefore, in case of applying the activated carbon to the atmospheric air-purifying device for a vehicle, it becomes an issue of how to reduce damage from PMs in the atmospheric air, especially PMs from the vehicle.